The spatial organization of differentiated cells in a field of other cell types often results in a distinctive pattern. Trichomes and stomata form a regularly spaced pattern over the surface of the shoot epidermis of Arabidopsis. While trichomes are dispensable structures, lending themselves to mutational analysis, stomata are functionally important for gas exchange. How stomata are specified and how their spacing is regulated is not well understood. l propose to study these questions using a transposon- mediated enhancer detection method in Arabidopsis. Already l have isolated three lines that each reveals a stomata-specific enhancer. Among these, one line exhibits clustering of stomatal cells as a putative stomatal mutant. l will search for more stomatal mutants and stomata-specific expression lines in our transposant collection. Genetic interactions between these mutants and previously characterized mutants affecting epidermal patterning will be determined to establish a hierarchy of genes involved in stomatal patterning. Molecular cloning of the mutant gene causing clustering of stomatal cells and other stomata-specific genes will allow further explorations of the molecular mechanisms governing pattern formation in this model system, via sequence analysis, overexpression, and genetic mosaics.